Window milling is commonly used in the oil and gas industry to change the direction of an existing cased wellbore. A window may be milled, for example, in an existing casing string to enable a driller to sidetrack around the lower portion of the original well which is to be abandoned. With the evolution of extended reach and horizontal drilling technology, window milling has also been used in drilling multilateral wells. These multilateral wells allow a pre-existing well to reach new production zones for extracting oil and natural gas. This is accomplished by milling away a portion of the casing in such a way and an amount that a new wellbore can be drilled through the window. This technique may be repeated at different locations in the well until the well has several lateral wellbores extending into one or more production zones.
Horizontal or highly deviated wells and extended reach wells can present torque and drag related problems as the drill string lays on the low side of the casing. In such cases, it is difficult to transmit a constant rotation and weight from the surface to the milling bit. The present invention is directed to methods and apparatus for transmitting more accurate and constant rotation and weight to the milling bit, independently from hole configuration and geometry. The present invention differs from the prior art in that the milling bit is guided into a predetermined location before milling is begun. After the milling has started the bit maintains its position since the downhole motor it is attached to is secured to the whipstock.
It is a common problem in the prior art for downhole motors to generate reactive torque, which can cause the motor to jump out of the whipstock. This causes significant gyration in torque, damages the whipstock, motor and mill, and tears up the casing. With high deviation and extended reach wells, it is also difficult to accurately use surface parameters, such as the rotational speed and the weight on the bit, to control the window milling. The steel cuttings produced from milling the wellbore casing should be small enough to be easily circulated out of the wellbore, thus ensuring that the cuttings are not left in the hole or on the whipstock. Controlling the parameters downhole so that more uniform (i.e. small) cuttings are produced enables easy circulation of the cuttings out the well. The present invention addresses these problems by providing a window milling apparatus which utilizes a guide or rail to control the position of the milling bit with respect to the whipstock.